The newspaper, citing two unnamed sources familiar with the matter, reported that the grand jury had begun its work in the US capital Washington “in recent weeks.”

The move is a sign that the sweeping federal investigation – which includes allegations that Trump campaign officials coordinated with Russia to tilt the election in the Republican’s favour – is gathering pace.

One source briefed on the matter said Mueller was investigating whether, either at the meeting or afterward, anyone affiliated with the Trump campaign encouraged the Russians to start releasing material they had been collecting on the Clinton campaign since March 2016.

Another source familiar with the inquiry said that while the president himself was not now under investigation, Mueller’s investigation was seeking to determine whether he knew of the meeting in advance or was briefed on it afterward.

A grand jury is a group of ordinary citizens who, working behind closed doors, considers evidence of potential criminal wrongdoing that a prosecutor is investigating and decides whether charges should be brought.

The establishment of a grand jury will allow Mueller – a former FBI director – to subpoena documents and get sworn testimony. It could lead to criminal indictments.

“You don’t impanel a grand jury unless your investigation has discovered enough evidence that you feel reflects a violation of at least one, if not more, criminal provisions,” he said. “If you secure an indictment, your next step is to arrest the defendant.”

At a rally in Huntington, West Virginia, on Thursday night, Trump said: “Most people know there were no Russians in our campaign." "We didn’t win because of Russia. We won because of you.”

Presidential lawyer Ty Cobb said he was not aware that a grand jury had been convened. “Grand jury matters are typically secret,” Cobb said, adding that “the White House favours anything that accelerates the conclusion of his work fairly.” “The White House is committed to fully cooperating with Mr Mueller.”

A lawyer for Trump, Jay Sekulow, appeared to downplay the significance of a grand jury, telling Fox News: “This is not an unusual move.”

White House spokeswoman Sarah Huckabee Sanders said the President was not likely the subject of an investigation.

“Former FBI director Jim Comey said three times the President is not under investigation and we have no reason to believe that has changed,” she said.

Trump has repeatedly denied allegations of collusion, saying he is the victim of a political “witch hunt” and “fake news.”

But the White House has been forced to acknowledge that Trump’s eldest son, Donald Trump Jr, his son-in-law Jared Kushner and his then campaign adviser Paul Manafort did meet a Kremlin-connected lawyer to get dirt on Hillary Clinton.

Lawyers for Trump Jr and Kushner did not immediately respond to requests for comment from Reuters.

Mueller is also said to be investigating financial records of Trump associates unrelated to Russia or the election, CNN reported.

The US broadcaster reported that federal investigators have seized on Trump and his associates' financial ties to Russia as one of the most fertile avenues for moving their probe forward, according to people familiar with the investigation.

The web of financial ties could offer a more concrete path towards potential prosecution than the broader and murkier questions of collusion in the 2016 campaign, these sources said, according to CNN.

Sources also told AFP described an investigation that has widened to focus on possible financial crimes, some unconnected to the 2016 elections, alongside the ongoing scrutiny of possible illegal coordination with Russian spy agencies and alleged attempts by Trump and others to obstruct the FBI investigation.

Trump has publicly warned Mueller that his financial dealings should be out of bounds and investigating them would cross a red line.

The revelation will only fuel speculation that Trump may try to curb the investigation by firing Mueller.

Two US senators introduced a bipartisan Bill on Thursday to pre-empt that move by insulating Mueller.

The legislation, sponsored by Democrat Chris Coons and Republican Thom Tillis, would bar a president from directly firing the special counsel without a judicial review.

Under the Bill, Mueller would be allowed to challenge his removal in court in the event he is fired without good cause.

“A back-end judicial review process to prevent unmerited removals of special counsels not only helps to ensure their investigatory independence, but also reaffirms our nation’s system of check and balances,” Tillis said in a statement.

Coons added: “Ensuring that the special counsel cannot be removed improperly is critical to the integrity of his investigation.”

The Straits Times

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